1. Brief Description of the Invention
The invention relates to a method for dewatering a two-phase liquid mixture of a thermoplastic resin melt and an aqueous phase in a counter-rotating twin screw extruder and withdrawing the aqueous phase from the extruder in liquid form. As a rule, the method is part of a method for dewatering a latex of a thermoplastic resin in a counter-rotating twin screw extruder.
In the dispersion employed, the synthetic resin is in the form of latex particles uniformly distributed in the continuous aqueous phase. The primary particles of the latex coagulate under the influence of shear forces in the extruder at a temperature in the thermoplastic region of the resin and form a molten phase which is mixed with the aqueous phase. In the screw extruder, a phase separation occurs due to a pressure gradient. The pressure gradient is caused by the construction characteristics of the extruder, e.g. decreasing screw pitch in the direction of transport or decreasing available cross-sectional area because of increasing core diameter or decreasing cylinder- and screw-diameter or by a restriction zone having kneading or mixing elements located after a section of the screw. The pressure gradient causes a back flow in which the aqueous phase, because of its lower viscosity, reaches a greater flow velocity than does the melt phase. Thus, there is a preferred back flow of the aqueous phase. The phase separation is caused by this effect: the melt phase accumulates in the zone of highest pressure while the aqueous phase flows back more strongly in the zone of least pressure. However, phase separation is hindered if the back flowing water collects as a continuous aqueous phase because it is then repeatedly mixed back into the resin melt by the action of the screws.
2. Description of the Art
It has long been known to obtain a melt of a synthetic resin by coagulation and dewatering of a resin latex in an extruder and optionally to mix the resin at the same time with a different thermoplastic resin. According to DE-A 20 37 784, dewatering takes place in three stages with decreasing pressure of the vapor phase. According to DE-A 22 43 696, coagulation is carried out under a pressure at which the separated water remains liquid. The resin melt is conveyed downstream from the mixture of resin melt and the separated aqueous phase with the build up of a pressure gradient and the water found in the resin is pressed out and transported backwards. A portion of the mixture of melt and water penetrates into an outlet for water removal located on the side of the extruder, from which the melt portion is transported back by means of a screw conveyor while the aqueous phase exits through a valve which maintains the pressure
The separation of the aqueous phase in liquid form before evaporation of the entire aqueous phase has the advantage that dissolved components, such as coagulating agents or emulsifiers, are removed at the same time and that the expenditure of energy for evaporation of the water and for pumping off large volumes of steam are saved.
According to U.S. Pat. No. 4,136,251, a twin screw extruder having a tightly sealed chamber is used. The mixture of melt and water is transported through a narrow region in a decompression zone of the extruder, from which the water emerges at moderate pressure through an overhead port equipped with a vent screw. The remaining melt is passed into a further pressure zone for complete dewatering and conveyed from there into a degassing zone. U.S. Pat. No. 4,148,991 teaches a method variant in which a synthetic resin latex alone is dewatered, without an added melt of a thermoplastic synthetic resin.
It is common to all the described methods that the mixture of resin melt and the separated aqueous phase is transported into a pressure zone which extends over the length of several screw channels from which the water is more strongly pressed with increasing pressure and is transported backwards. The screw channels are filled with this melt-water mixture, in which the water portion decreases in the direction of increasing pressure. The longer such a pressure zone is, the more water is constantly pressed back into the molten phase so that complete dewatering is attained only with difficulty.
The removal of water through a lateral channel equipped with a conveyor screw must take place in a region in which the water forms the continuous phase and the melt is the phase distributed therein. If the lateral channel is too close to the site at which the latex enters, considerable amounts of the as-yet uncoagulated latex resin are lost with the water which has been expressed. Thus, the water is removed in a zone which is located as far as possible downstream. However, the phase reversal boundary, where the melt-in-water mixture converts to a water-in-melt mixture, is not in a fixed location because of the instability of the transport effect during continuous operation. If it retreats to the starting point of the dewatering channel, melt presses in strongly and can sometimes not be held back by conveyor screws, or only with difficulty. The operation of a dewatering extruder thus demands continuous careful monitoring and must often be interrupted for obviating disturbances despite careful attention.